Use of mobile communications networks has tremendously increased over the last decade. Operators of the mobile communications networks have increased a number of base stations in order to meet an increased demand for service by users of the mobile communications network. The base stations typically comprise radio systems for relaying radio signals, including software and hardware components. The radio signals are typically relayed into a cell of the mobile communications network. The operators of the mobile communications network wish to reduce the costs of the base stations. It is one option to implement the radio system embedded within an antenna array in order to reduce the costs of the base station. For the radio system embedded in the antenna array components of the radio system are implemented on a chip. Implementing the radio system embedded within the antenna array reduces real estate needed to house the hardware components of the base station. Substantially all of the hardware components of the radio system are implemented embedded in the antenna array. The amount of the real estate to house the radio system is thereby reduced. Power consumption during normal operation of the radio system is substantially reduced when implementing the radio system embedded within the antenna array.
It is of interest to provide a reliable quality of service to an individual user of the mobile communications network given the increase in the number of users. Several techniques have been suggested in order to deal with the increased number of users within the mobile communications network. One of the several techniques comprises beam forming capabilities in order to direct a beam relayed by the radio system in different directions to improve service coverage within the cells of the mobile communications network. The beam forming techniques rely on defined phase and amplitude relations between several ones of the antenna elements of the active antenna system. Calibration of transmit paths and receive paths is required to provide the defined phase and amplitude relationship between the beams. The calibration allows the estimation of a phase and amplitude deviation accumulated along the transmit path of the radio system. Likewise the calibration comprises estimating a phase and amplitude deviation accumulated along the receive paths of the radio system. In a second step the phase and amplitude deviation accumulated along the transmit paths can be corrected. An appropriate phase and amplitude change may be applied to the individual ones of the transmit paths to yield the defined phase and amplitude relationship between the individual ones of the transmit paths of the radio system in order to allow for beam forming techniques.
A technique of analogue or digital predistortion is known in the art. It is to be understood that the present invention may be combined with an analogue or digital predistortion technique in order to linearise transfer characteristics of the radio system. Such a combination is disclosed in a co-pending application of the applicant Ser. Nos. 12/416,596 and 12/416,626 .
The present invention may be implemented as part of a radio system monitoring a radio frequency (RF) transmit power in order to help the beam forming techniques. Details of such a combination are disclosed within a co-pending application of the applicant Ser. No. 12/416,630 .
In the prior art it is common to use a calibration signal generator in order to provide a calibration signal. The calibration signal is used to calibrate the phase and amplitude changes applied to the transmit path in order to obtain the defined phase and amplitude relation between the transmit paths. Typically the calibration signal is hidden within a payload signal that is to be relayed along the transmit paths. Alternatively a dedicated calibration signal may be used during idle times of the radio system. A disadvantage of the calibration signal being applied during idle times of the radio system is twofold. Firstly the calibration signal is visible to other radio systems and to the other users within the cell of the mobile communications network. Therefore signal-to-noise ratio (SNR) for the other radio systems and/or the users present within the cell would be deteriorated. Burying or hiding the calibration signal within the payload signals is common practise. Hiding the calibration signal overcomes the disadvantage of unwanted calibration signals being relayed and visible to the other radio systems and/or the users within the cell of the mobile communications network. A disadvantage of the hiding of the calibration signal is the low SNR of the hidden calibration signal. Hence one is confronted with very poor SNR values when comparing the original calibration signal with the calibration signal hidden in the payload signal that has travelled along the transmit paths. Consequently the low SNR of the hidden calibration signal makes the calibration using the hidden calibration signal difficult.
US 2003/0236107 to Tomohiro Azuma provides a calibration system for an antenna array receiving apparatus as used for cellular mobile communication systems. In the antenna receiving apparatus of the Azuma calibration system, a calibration time determining unit determines an adaptive calibration time based on detection voltages of the total reception power inputted to antenna radio receiving units. A calibration signal processing unit detects phase and amplitude information of a calibration signal. The Azuma calibration system uses a dedicated calibration signal for carrying out the calibration. The calibration signal within the Azuma system is further hidden within a payload signal being relayed by the antenna system.